Accessibility: Make vote arrows into buttons.

This adds three attributes to vote arrows:

* role="button" is part of ARIA and tells user agents that even though
  the vote arrow is a div, it's actually meant to be a button.
* aria-label, another part of ARIA, this tells user agents what to
  call the newly buttonized.
* tabindex, this makes the buttons able to receive keyboard focus so you
  tab through the document.

r2/r2/templates/printable.html

@@ -148,6 +148,13 @@ ${self.RenderPrintable()}
        %else:
          onclick="$(this).show_unvotable_message()"
        %endif
+       role="button"
+       % if dir > 0:
+       aria-label="${_("upvote")}"
+       % else:
+       aria-label="${_("downvote")}"
+       % endif
+       tabindex="0"
        >
   </div>
 </%def>